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Appendix II7.

923

P. 4fdp.  Redundant   Members. — It  has  already  beer*
mentioned  that  the static" force diagram shews no stress in a

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redundant or superfluous member; but as soon, however, as
the structure is subjected to deformation, and not till then, such a
member receives stress on account of the strain in the remaining
bars, the static stresses in the latter being at the same time more
or less altered. The problem, therefore, can only be solved by
reference to the work done on the bars. Now when there are no
redundant members in a structure

no. of members = 2 (no. of joints) -3

a rule easily verified, and useful for discovering the number of
extra bars.

Having ascertained these, there are two methods of finding
the stresses set up in them.    Assuming the whole structure to be